1. Field of the Invention
The present invention relates to apparatus and a method for making balls on fine wires used in automatic ball wire bonders. More particularly, the present invention relates to a method for making an air ball of a predetermined size on any of a plurality of different types and sizes of fine bonding wires.
2. Description of the Prior Art
Automatic wire bonders require that numerous parameters be set prior to start of automatic wire bonding operations in automatic wire bonding machines. A typical example of a state of the art automatic wire bonder is a Model 1488 XQ made by Kulicke and Soffa Industries, Inc. in Willow Grove, Pa. 19090. One of the programmable displays or screens which may be selected for display on the monitor permits selection of a "Ball size ratio" and entry of the wire size in mils to be used to make the ball. It is generally assumed that the air ball size becomes the product of the wire diameter times the ball size ratio in these state of the art prior art machines. The software programmed into the wire bonder must determine the amount of wire to be melted to produce the desired ball. It can be shown that the volume of the ball is equal to 6/.PI..times.D.sub.B.sup.3 where D.sub.B is the ball diameter. Also the length of the wire melted to produce the ball is equal to 4/.PI..times.D.sub.W.sup.2 where D.sub.W is the diameter of the wire. In the state of the art wire bonders, a tail of wire is made which exceeds the length of wire needed to produce the ball programmed into the wire bonder. By establishing a desired or near optimum constant EFO current (I.sub.c) it is possible to calculate or determine empirically the time (T.sub.i) required to produce a ball for the ball size ratio programmed into the prior art wire bonders.
Programming optimum current values into an automatic wire bonder makes it relatively easy for an operator of the wire bonder to rapidly set parameters for bonding semiconductor chips that cover a broad spectrum of problems, however, such automatic selection effectively limits the operator from making needed adjustments that would be useful for improving bonding of fine wire interconnections. An example will illustrate the point. Capillary bonding tools used for fine pitch wire bonding require balls smaller than those presently recommended for standard capillary bonding tools. When a small EFO current is employed to make a small ball, the amount of spring back or elastic force in the loop was found to be reduced. It would be desirable to make small air balls for use with fine pitch capillaries such as those shown and described in U.S. Ser. No. 08/861,965 filed May 22, 1997 and U.S. Pat. No. 5,558,270, assigned to the same assignee as the present invention, and still be able to control elastic spring back in the fine wire interconnection loops.